Dislocation mechanisms and 3D twin architectures generate exceptional strength-ductility-toughness combination in CrCoNi medium-entropy alloy

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Abstract

Combinations of high strength and ductility are hard to attain in metals. Exceptions include materials exhibiting twinning-induced plasticity. To understand how the strength-ductility trade-off can be defeated, we apply in situ, and aberration-corrected scanning, transmission electron microscopy to examine deformation mechanisms in the medium-entropy alloy CrCoNi that exhibits one of the highest combinations of strength, ductility and toughness on record. Ab initio modelling suggests that it has negative stacking-fault energy at 0K and high propensity for twinning. With deformation we find that a three-dimensional (3D) hierarchical twin network forms from the activation of three twinning systems. This serves a dual function: conventional twin-boundary (TB) strengthening from blockage of dislocations impinging on TBs, coupled with the 3D twin network which offers pathways for dislocation glide along, and cross-slip between, intersecting TB-matrix interfaces. The stable twin architecture is not disrupted by interfacial dislocation glide, serving as a continuous source of strength, ductility and toughness.

Abstract

Materials that show twinning-induced plasticity can offer unusual combinations of strength and ductility. Here, authors study deformation twinning and dislocation behaviour in a medium-entropy alloy CrCoNi and find a three-dimensional (3D) hierarchical twin network that forms from the activation of three twinning systems.

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A fracture-resistant high-entropy alloy for cryogenic applications.

Bernd Gludovatz, Anton Hohenwarter, Dhiraj Catoor … (2014)

High-entropy alloys are equiatomic, multi-element systems that can crystallize as a single phase, despite containing multiple elements with different crystal structures. A rationale for this is that the configurational entropy contribution to the total free energy in alloys with five or more major elements may stabilize the solid-solution state relative to multiphase microstructures. We examined a five-element high-entropy alloy, CrMnFeCoNi, which forms a single-phase face-centered cubic solid solution, and found it to have exceptional damage tolerance with tensile strengths above 1 GPa and fracture toughness values exceeding 200 MPa·m(1/2). Furthermore, its mechanical properties actually improve at cryogenic temperatures; we attribute this to a transition from planar-slip dislocation activity at room temperature to deformation by mechanical nanotwinning with decreasing temperature, which results in continuous steady strain hardening. Copyright © 2014, American Association for the Advancement of Science.

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The conflicts between strength and toughness.

Robert Ritchie (2011)

The attainment of both strength and toughness is a vital requirement for most structural materials; unfortunately these properties are generally mutually exclusive. Although the quest continues for stronger and harder materials, these have little to no use as bulk structural materials without appropriate fracture resistance. It is the lower-strength, and hence higher-toughness, materials that find use for most safety-critical applications where premature or, worse still, catastrophic fracture is unacceptable. For these reasons, the development of strong and tough (damage-tolerant) materials has traditionally been an exercise in compromise between hardness versus ductility. Drawing examples from metallic glasses, natural and biological materials, and structural and biomimetic ceramics, we examine some of the newer strategies in dealing with this conflict. Specifically, we focus on the interplay between the mechanisms that individually contribute to strength and toughness, noting that these phenomena can originate from very different lengthscales in a material's structural architecture. We show how these new and natural materials can defeat the conflict of strength versus toughness and achieve unprecedented levels of damage tolerance within their respective material classes.

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Metastable high-entropy dual-phase alloys overcome the strength-ductility trade-off.

Zhiming Li, Konda Pradeep, Yun Deng … (2016)

Metals have been mankind's most essential materials for thousands of years; however, their use is affected by ecological and economical concerns. Alloys with higher strength and ductility could alleviate some of these concerns by reducing weight and improving energy efficiency. However, most metallurgical mechanisms for increasing strength lead to ductility loss, an effect referred to as the strength-ductility trade-off. Here we present a metastability-engineering strategy in which we design nanostructured, bulk high-entropy alloys with multiple compositionally equivalent high-entropy phases. High-entropy alloys were originally proposed to benefit from phase stabilization through entropy maximization. Yet here, motivated by recent work that relaxes the strict restrictions on high-entropy alloy compositions by demonstrating the weakness of this connection, the concept is overturned. We decrease phase stability to achieve two key benefits: interface hardening due to a dual-phase microstructure (resulting from reduced thermal stability of the high-temperature phase); and transformation-induced hardening (resulting from the reduced mechanical stability of the room-temperature phase). This combines the best of two worlds: extensive hardening due to the decreased phase stability known from advanced steels and massive solid-solution strengthening of high-entropy alloys. In our transformation-induced plasticity-assisted, dual-phase high-entropy alloy (TRIP-DP-HEA), these two contributions lead respectively to enhanced trans-grain and inter-grain slip resistance, and hence, increased strength. Moreover, the increased strain hardening capacity that is enabled by dislocation hardening of the stable phase and transformation-induced hardening of the metastable phase produces increased ductility. This combined increase in strength and ductility distinguishes the TRIP-DP-HEA alloy from other recently developed structural materials. This metastability-engineering strategy should thus usefully guide design in the near-infinite compositional space of high-entropy alloys.

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Author and article information

Journal

Journal ID (nlm-ta): Nat Commun

Journal ID (iso-abbrev): Nat Commun

Title: Nature Communications

Publisher: Nature Publishing Group

ISSN (Electronic): 2041-1723

Publication date (Electronic): 20 February 2017

Publication date Collection: 2017

Volume: 8

Electronic Location Identifier: 14390

Affiliations

[1 ]Center of Electron Microscopy & State Key Laboratory of Silicon Materials, Department of Materials Science & Engineering, Zhejiang University , Hangzhou 310027, China

[2 ]Department of Physics and Astronomy, George Mason University , Fairfax, Virginia 22030, USA

[3 ]Department of Materials Science and Engineering, Xi'an Jiaotong University , Xi'an 710049, China

[4 ]Materials Sciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, USA

[5 ]Materials Sciences & Technology Division, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, USA

[6 ]Department of Mechanical Engineering & Materials Science, University of Pittsburgh , Pittsburgh, Pennsylvania 15261, USA

[7 ]Department of Materials Science & Engineering, University of California , Berkeley, California 94720, USA

Author notes

[a ] yu_qian@ 123456zju.edu.cn

[b ] sxm2@ 123456pitt.edu

[c ] roritchie@ 123456lbl.gov

Author information

Bernd Gludovatz http://orcid.org/0000-0002-2420-3879

Robert O. Ritchie http://orcid.org/0000-0002-0501-6998

Article

Publisher Item ID: ncomms14390

DOI: 10.1038/ncomms14390

PMC ID: 5321736

PubMed ID: 28218267

SO-VID: e4e85e21-e34a-4613-9ddf-0d298ae342f0

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Dislocation mechanisms and 3D twin architectures generate exceptional strength-ductility-toughness combination in CrCoNi medium-entropy alloy

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High Entropy Alloys and Complex Concentrated Alloys

Most cited references 19

A fracture-resistant high-entropy alloy for cryogenic applications.

The conflicts between strength and toughness.

Metastable high-entropy dual-phase alloys overcome the strength-ductility trade-off.

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